CHICAGO (Reuters) - Smells so irritating they make you
cough or gag may act upon a single type of cell in the nose
that senses caustic chemicals and warns the brain of potential
danger, U.S. researchers said on Tuesday.

Scientists had thought such smells acted directly on nerve
endings in the nose, but the study in mice suggests special
cells in the tip of the nose act as air quality control sensors
that protect the body from harmful chemicals.

"You can imagine walking into an environment where there is
a lot of irritating dust in the area. This would give you
pause," said Thomas Finger of the University of Colorado
Denver, whose study appears in the Journal of Neurophysiology.

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Finger said these chemosensory cells are found in most
aquatic vertebrates, including sharks, bony fish and lampreys.
He thinks they are part of an ancient sensory system and they
are likely present in all mammals.

"The current study is the first in mammals that has a clear
idea of what these cells are responding to," Finger said in a
telephone interview.

"Some fish use them to detect predators," he said.

In people, the cells likely trigger a response to high
concentrations of irritating chemicals. Ammonia, paint thinner
or even the spray from opening a carbonated soda can set of the
alarm.

University of Colorado Denver researcher Diego Restrepo,
who also worked on the study, said high concentrations of
irritants can even trigger a reflex that causes you to stop
breathing for a few moments.

"This is one of these really hard-wired reflexes. It gives
you time to get out," Restrepo said in a telephone interview.

The researchers used nasal tissue from mice to measure
changes in chemosensory cells as they exposed them to low and
high levels of several irritating, volatile chemical odors.

They saw evidence that the cells not only responded to the
stimuli but that they were relaying that information to nerve
fibers in the nose.

And they said it takes more than a mere whiff of an
offending odor to trigger the response. Restrepo said only
potentially dangerous levels of odors can set off the
protective gagging-and-coughing response.

"There are some people who are especially sensitive to
these irritants. This could have implications for their
treatment," Restrepo said.

The study was funded by the National Institute on Deafness
and Other Communication Disorders, part of the U.S. National
Institutes of Health.